the PYTHONPATH must be set to the weewx 'bin' directory. in your case, that would be '/usr/share/weewx'.
given that your current directory is /usr/share/weewx, you can use either a relative path:
PYTHONPATH=..
or you can use the absolute path:
PYTHONPATH=/usr/share/weewx
alternatively, you can use absolute path for everything, and thus be independent of where you do the invocation:
PYTHONPATH=/usr/share/weewx /usr/share/weewx/user/interceptor.py ...